The present invention relates to a light guiding plate comprised of a transparent plate having on its one side a multiplicity of grooves or ridges on which rays of light emitted from light sources, such as fluorescent lights or light emitting diodes, are diffusely reflected, and more particularly, to a light guiding plate ensuring a uniform luminance throughout the entire light guiding plate.
In the case of a back lighting means or a back light for a liquid crystal display, it is desirable that the rays of light be diffused as uniformly as possible for effective view of the display. Recently proposed and put into practical use, as such a light diffusing measure is a method in which a transparent plate used for transmission of light is used as the light diffusing means.
FIGS. 14 and 15 depict a specific configuration of a lighting means laid open to public inspection under Provisional Publication Nos. 165504/90 and 176692/90.
The lighting means comprises a plate material (hereinafter referred to as "light guiding plate"), designated at reference numeral 50, formed from a transparent material such as acrylic resin or other plastic and glass. The light guiding plate includes on a first side a multiplicity of grooves 51 having a substantially V-shaped section and extending parallel to a width direction of the light guiding plate 50 (See FIG. 15). The lighting means further comprises a pair of light sources 52 and 53, such as fluorescent lights, disposed on both lateral sides of the light guiding plate 50, and a reflection plate 54 arranged parallel and close to the first side of plate 50. Rays of light emitted from the light sources 52 and 53 travel through the grooves 51 of the light guiding plate 50 and the auxiliary rear reflection plate 54, and emerge from the face 50a of the light guiding plate 50 to illuminate an object. The lighting means of this type provides relatively satisfactory functions, producing a relatively uniform reflection of light on the grooves and reducing attenuation of light emitted from the light sources.
FIGS. 16 to 18 depict more specifically the characteristics of the above lighting means. Referring first to FIG. 16, rays of light L1 from the light source 52 impinge on a slanted surface 51a of the groove 51 in the form of parallel rays, most of which are reflected as indicated by arrows into the light guiding plate 50 for emittance from the face 50a. Rays of light L2 from the light source 53 fall on a slanted surface 51b adjoining the slanted surface 51a of the light guiding plate 50 and, like the rays of light L1, are internally reflected into the light guiding plate 50. In this case, if an angle .alpha.1, which the slanted surfaces 51a and 51b form with respect to the reverse side 50b of the light guiding plate 50, is more than 45 degrees, the reflected rays of light are directed outwardly around groove bottoms 51c in a diverging manner. This results in a reduction in the amount of light emerging from regions of the face 50a of the light guiding plate 50 which correspond to the groove bottoms 51c. As seen from the upper diagram representing the luminance BR on the face 50a of the light guiding plate 50, the luminance of these regions will be lowered and hence striped low-luminance portions will appear on the face 50a of the light guiding plate 50 corresponding to the bottoms 51c of the grooves 51. Although such low-luminance portions do not actually occur as sharp as depicted, since the rays of light L1 and L2 emitted from the respective light sources 52 and 53 are not completely parallel rays, the light pattern produced will disadvantageously permit perceptible striped low-luminance portions to appear.
FIG. 17 depicts a case adverse to the above, in which an angle .alpha.2 which the slanted surfaces 51a and 51b of the groove 51 form with respect to the reverse side 50b of the light guiding plate 50 is less than 45 degrees. In this case, the rays of light L1 and L2 will be inwardly reflected toward the groove bottoms 51c in a converging manner. Consequently, as opposed to the above-described case, the luminance of the portions of the face 50a of the light guiding plate 50 corresponding to the bottoms 51c will become higher than that of their peripheries.
In this manner, the configuration shown in FIGS. 16 and 17 will basically result in the occurrence of optical unevenness. Thus, the conventional technical development has emphasized the reduction of the possible unevenness of luminance to a degree imperceptible with unaided eye, on the assumption that it is impossible to completely prevent this optical unevenness from occurring. More specifically, the number of grooves to be formed is increased to lessen the intervals of the grooves correspondingly to which the luminance will vary on the face of the light guiding plate, whereby the lower and higher luminance portions are optically combined with unaided eye to allow the entire light guiding plate to be perceived to have relatively uniform luminance. Inconveniently, this will necessitate a minute grooving operation mainly including a skiving step, thus leading to a significantly lower productivity and hence a higher cost of the light guiding plate.
In view of the above, if the angle which the slanted surfaces 51a and 51b of the groove 51, formed with respect to the reverse side 50b of the light guiding plate, is substantially 45 degrees, it might be expected a satisfactory result will be achieved. In fact, however, this will also cause a lighting unevenness due to the following reasons.
Referring to FIG. 18, an angle .alpha.3, that the slanted surfaces 51a and 51b of the groove 51 form with respect to the reverse side 50b of the light guiding plate, is substantially 45 degrees. In consequence, rays of light L1 and L2 striking on the slanted surfaces 51a and 51b are substantially orthogonally reflected and emerge from the face 50a of the light guiding plate in the form of the parallel rays L1 and L2. More specifically, the rays of light emerging from the face of the light guiding plate tend to be separated along the groove bottom 51c into two striped regions of rays of light L1 and L2. In this case, the groove located closer to the light source 52 as shown, for example, will receive rays of light L1 traveling through the light guiding plate with a lesser amount of attenuation, as well as rays of light L2 traveling long distance through the light guiding plate with a greater amount of attenuation, which results in a difference in the amount of light between the two rays of light L1 and L2. Thus, two optical regions having different amounts of attenuation will alternately form relatively distinctly on the face 50a of the light guiding plate 50 as discussed above, and the resultant difference in the amount of light will directly lead to a difference in the luminance BR on the face of the light guiding plate.